Electron discharge device having lossy resonant elements disposed within the electromagnetic field pattern of the slow-wave circuit



Dec. 26, 1967 R. R. RUBERT 3,360,679

ELECTRON DISCHARGE DEVICEYHAVING LOSSY RESONANT ELEMENTS DISPOSED WITHINTHE ELECTROMAGNETIC FIELD PATTERN OF THE SLOW-WAVE CIRCUIT Filed Feb.21, 1964 2 Sheets-Sheet 1 INVENTOR.

' RODNEY R. RUBERT ATTORNEY 1366- 1967 R. R. RUBERT ELECTRON DISCHARGEDEVICE HAVING LOSSY RESONANT ELEMENTS DISPOSED WITHIN THEELECTROMAGNETIC FIELD PATTERN OF THE SLOW-WAVE CIRCUIT 2 Sheets-Sheet 2Filed Feb. 21, 1964 FIG. 6

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INVENTOR RODNEY R. RUBERT United States Patent ELECTRON DISCHARGE DEVICEHAVING LOSSY RESONANT ELEMENTS DISPOSED WITHIN THE ELECTROMAGNETIC FIELDPATTERN OF THE SLOW-WAVE CIRCUIT Rodney R. Rubert, Santa Clara, Calif.,assignor to Varian Associates, Palo Alto, Calif., a corporation ofCalifornia Filed Feb. 21, 1964, Ser. No. 346,495 21 Claims. (Cl.315-3.5)

ABSTRACT OF THE DISCLOSURE Undesired electromagnetic wave energyassociated with certain frequencies, primarily at the bandedge regionsof the operating passband characteristic of a traveling wave type ofhigh frequency electron discharge device and also at frequencies abovethe operating passband characteristic such as, for example, thoseassociated with higher order modes can be dissipated with theutilization of lossy resonant elements disposed internally of a slowwave circuit at positions removed from the interaction region so as tominimally perturb electromagnetic wave energy associated withfrequencies within the operating band.

The lossy resonant elements can be disposed to primarily perturb higherorder modes while simultaneously being tuned to another undesiredfrequency for resonant absorption of energy e.g., the bandedge of theoperating passband characteristic. In all cases improved tube stabilityagainst undesired oscillations results.

This invention is concerned in general with high frequency travelingwave tubes, and more particularly, with such tubes having oscillationsuppression means incorporated therein.

High frequency electron discharge devices such as the traveling wavetube are finding increased usage in applications such as frequency agileradar systems, phased array radar systems and as broadband amplifiers.However, as R.F. powers involved in such systems approach the kilowattand megawatt level, spurious oscillations such as non-resonant driveinduced, pulse induced and higher order mode types and especiallyoscillations of the resonant circuit type become increasinglydeleterious to high efiiciency and stabilized operation. Such resonantcircuit oscillations are generally characterized by modes, either higherorder or fundamental going into oscillation at that portion of therespective :mode which is characterized by a low or zero group velocity.Such oscillations are the result of numerous factors such as, forexample; drive induced oscillations which are caused by operating thetraveling wave tube above saturation in order to take advantage of therather flat power output characteristics of an overdriven tube whereinthe overdriven tube iscaused to oscillate due to the slowing down of theelectron beam caused by the heavy extraction of beam energy with aresultant synchronization of beam velocity and phase velocity of the RF.energy at the upper band edge of the fundamental mode; pulse inducedoscillations which are caused by the sweep of the beam voltage vto itsoperating point also induce resonant circuit oscillations at the bandedges of the fundamental mode. Both the driveinduced oscillations andthe pulse induced oscillations generally occur at the band edges of theoperating mode or that part of the mode which has a low or zero groupvelocity.

Other types of spurious oscillations which preclude optimum stabilityare oscillations such as the higher order mode oscillations. Thesehigher order mode oscillations can be especially troublesome, mainlybecause the slow- 3,360,679 Ce Patented Dec. 26, 1967 wave interactioncircuit is difficult to properly terminate for all mode configurations.Since it is essential that the fundamental mode of operation be wellterminated, major emphasis is placed on this mode. The impedancematching terminating configuration resulting in exceptional terminatingcharacteristics for one mode will not in general properly terminateother modes which will in general result in resonant behavior existingin these other modes since diverse field patterns exist for the variousmodes. The fundamental or lowest order of mode of propagation for anyperiodic slow wave circuit is characterized by a particular fieldpattern in a plane transverse to the direction of propagation whichfield pattern is periodic in amplitude along the axis of propagation.Higher order modes of propagation are herein defined as any modes otherthan the fundamental mode which are also characterized by a particularpattern in a plane transverse to the direction of propagation whichfield pattern is independent of position along the axis of propagationand which field patterns are each individually distinct and differentfrom each other.

Stability or the absence of output R.F. energy in the absence of RF.input energy and dependence of frequency and amplitude of the outputenergy on the frequency and amplitude of the input energy becomes apronounced factor in limiting power level and overall gain of travelingwave tubes at high power levels. Energy loss to high order modes limitsefliciency for the fundamental or operating mode, thereby providing alimitation on power level and gain. The adverse effects of drive inducedor pulse induced oscillations are apparent in the input and outputportions of non-stabilized traveling wave tubes. Such oscillations cancause carbonization of the insulation utilized on the input portion ofthe circuit or such oscillations can couple to the output portion of thecircuit and are particularly undesirable at this point when the tube isutilized, for example, in a frequency agile radar system since such bandedge oscillations can provide undesired identfying signals in additionto the desired signal. The power levels involved in such band edgeoscillations can be quite high as, for example, on the order of /3 to /2the peak power output on the main pulse of the tube.

It is desirable for various reasons, such as flat power outputcharacteristics over the frequency band of operation with constant drivepower to operate a high powered traveling wave tube in an overdriven oroversaturated condition. However, as mentioned previously, formerly whentraveling wave tubes were operated in overdriven or oversaturatedconditions, oscillations resulted. Such oscillations were present bothwithin the operating band and at the band edges. The present inventionprovides a novel approach to eliminating spurious oscillations such asmentioned previously in traveling wave tubes. The novel solutionemployed by the present invention to obtain stability in traveling wavetubes is the utilization of lossy resonant elements internal to theslow-Wave interaction circuit of the traveling wave tube. The lossyresonant elements employed by the present invention can take severalforms such as, for example, resonant conductive loops having lossymaterial deposited thereon, resonant cavities either of the Waveguide orcoaxial variety having lossy material deposited therein and positionedinternally of the slow-wave interaction circuit. The aforementionedlossy resonant elements have the advantage by being positioned internalto the slow-wave interaction circuit of not'radiating energy externallyof the tube while simultaneously providing the desired stabilization.The resonant lossy elements disclosed in the present invention areparticularly useful in the cloverleaf type slow-wave type of circuit. Anexplanation of the theoretical aspects of the cloverleaf slow wavecircuit can be found in the Proceedingsof the I.R.E., August 1957, pages1112 to 1118 and o in the I.R.E. Transactions on Military Electronics,April 1961, pages 39 to 45, as Well as in other publications.

It. is therefore, the object of the present invention to provide novelstability techniques for traveling wave tubes.

One feature of the present invention is the provision of a travelingwave tube having novel oscillation suppression means therein.

One feature of the present invention is the provision of a travelingwave tube having at least one lossy resonant element disposed thereinfor the purpose of suppressing oscillations in said traveling wave tubeat at least one frequency.

Another feature of the present invention is the utilization in atraveling wave tube of conductive resonant elements having lossymaterial deposited thereon wherein said lossy resonant elements arepositioned such as to suppress undesired oscillations in said travelingwave tube.

Anotherfeature of the present invention is the provision of a travelingwave tube having a plurality of lossy resonant elements disposed thereininternally of the slowwave interaction circuit said plurality of lossyresonant elements being adapted and arranged such as to suppressoscillations in that portion of the operating mode which ischaracterized by a low or zero group velocity.

Another feature of the present invention is the provision of a travelingwave tube having a plurality of lossy resonant elements disposed thereininternally of the slowwave interaction circuit, said plurality of lossyresonant elements being adapted and arranged to suppress higher modes ofpropagation in said traveling wave tube device.

Another feature of the present invention is the provision of a travelingwave tube having a plurality of lossy resonant elements disposed thereininternally of the slow-wave interaction circuit, said plurality of lossyresonant elements being adapted and arranged to suppress oscillations inboth higher order modes of propagation and in that portion of theoperating mode which is characterized by a low or zero group velocity.

Another feature of the present invention is the provision of a travelingwave tube having a plurality of lossy resonant elements disposed thereininternally of the slow-wave interaction circuit, said lossy resonantelements being positioned in said traveling wave tube at portionsthereof which are characterized by having low electromagnetic fieldintensity of'the operating mode.

Another feature of the present invention is the particularization in theaforementioned features of the lossy resonant element as being aconductive loop having lossy material deposited thereon.

Another feature of the present invention is the particularization of thelossy resonant element in any of the-aforementioned features as being acavity resonator having lossy material deposited thereon.

Another feature of the present invention is the particularization of thelossy resonant element mentioned in any of the aforementioned featuresas being a coaxial cavity resonator having lossy material depositedthereon.

Other features and advantages of the present invention will become moreapparent upon'a perusal of the following specification taken inconjunction with the accompanying drawings wherein:

FIG. l is a fragmentary longitudinal cross-sectional view, partly inelevation, of a high power traveling wave tube incorporating certain ofthe'novel features of the present invention;

FIG. 2 is an enlarged cross-sectional view of the traveling wave tubedepicted in FIG. 1 taken along the lines 22 in the direction of thearrows;

FIG. 3 is a fragmentary cross-sectionalview depicting an alternativeembodiment of the present invention;

FIG. 4 is a cross-sectional view of the resonant element depicted in thealternative'embodiment of FIG. 3 taken-along the lines 4-4 in thedirection of the arrows;

FIG. 5 is a fragmentary cross'sectional view of another alternativeembodiment of the present invention showing a coaxial resonator type oflossy circuit element.

FIG. 6 is a cross-sectional view taken along the lines 6-6 in thedirection of the arrows of the alternative embodiment depicted in FIG.5;

FIG. 7 is an illustrative w-fi diagram of the cloverleaf slow-Wavecircuit depicted in FIGS. 1, 2, 6;

FIG. 8 is an illustrative graphical portrayal of a voltage pulse and thepulse induced oscillations encountered in non-stabilized traveling wavetubes as the pulse sweeps through its operating range;

FIG. 9 is an illustrative graphical portrayal of an R.F. output pulse inoverdriven conditions showing the effects of drive induced oscillationsboth in the passband and at the band edges without the utilization ofthe novel oscillation suppression techniques of the present invention.

FIG. 10 is an illustrative graphical portrayal of power v. time of anRF. output pulse with the utilization of the oscillation suppressiontechniques of the present invention.

Referring now to the drawings, there is shown in FIG. 1 a traveling wavetube 12 of the aforementioned cloverleaf slow wave circuit design 13having an electron gun portion 14 disposed at the one end thereof,together with accelerating anode portion 15 and a collector structure 16disposed at the downstream end. R.F. input coupler 17 and cooling means18 of conventional design are shown in elevation in FIG. 1. Since theparticular details of the mechanical features of the tube do not formpart of the present invention and can be found elsewhere a detaileddescription will not be given. For further information on the particulardetails of a traveling wave tube of the type depicted in FIG. 1, seeU.S. patent application Ser. No. 56,415 filed Sept. 16, 1960, by John A.Ruetz et al., assigned to the same assignee as the present invention.Since the general operation of traveling wave tubes is well known, adetailed explanation thereof will not be presented herein. Suffice it tosay that the slow-wave interaction circuit structure 13 supports a highfrequency R.F. field that interacts with the electron beam produced bythe gun portion 14 of the tube such that useful interaction resultstherefrom.

In brief, the particular slow wave circuit depicted in FIGS. 1-6comprises a plurality of circular periodic sections of cloverleafconfigurations, one of which is shown in detail in FIG. 2 positioned inhollow cylindrical shells 19. The cloverleaf sections each include twometallic end walls 20, common to adjacent sections, each end wall havingan annular beam aperture 21 axially positioned therein, which alsoserves as a capacitive coupling opening between sections. A sinuousorfour-element cloverleaf shaped metallic side wall 22 is brazed betweenthe two end walls 20 of each section. The common walls 20 separating thecavity section are provided with a plurality of radially disposedconductive coupling slots 23 spaced apart every 45 relative to eachother such that every other section is in alignment. The particular typeof slow-wave section utilized in this traveling wave tube amplifier isdescribed in U.S. patent application, Serial No. 7,481, entitledConductive Coupling Means and Methods for High Frequency Apparatus,filed February 8, 1960, as a continuation of Serial No. 536,597, filedSeptember 26, 1955 by Marvin Chodorow. The coupling between theslow-wave sections is termed negative mutual inductive coupling, whichgives the slow-wave structure a forward wave fundamental mode, and istherefore a higher impedance structure than certain other types ofslow-wave structures. High impedance permits the attainment of a highelficiency for this traveling wave tube amplifier.

As can be seen from examination of FIG. 2, the sinuous side walls 22 ofeach cloverleaf section form a plurality, four in number for the tubeshown in FIG. 1, of spaced hollowed-out chambers 25 for each maincloverleaf section. The hollowed-out chambers 25 are 90 spaced rotatedwith respect to each other. The H-fields of the fundamental operatingmode depicted by the dashed lines 26, as can be best seen in FIG. 2,generally follow the curvature of the sinuous side walls 22. It isreadily apparent upon examination of the I-I-fields represented in FIG.2 for the TM mode mode that the intensity thereof is minimal or low forthe fundamental mode at the peripheral wall portion 32 of each chamber25. Whereas higher order modes can generally be Said to have high ormaximized H as well as E fields in the vicinity of the leaves orchambers 25 such as shown by the illustrative higher order modesrepresented by the dot-dash lines 26'. Lossy resonant elements 24 arepositioned in each cloverleaf section in chambers 25 as shown in FIGS. 1and 2. The lossy elements 24 as shown in FIGS. 1 and 2 take the form ofU-shaped loops and can be made of any highly conductive material, suchas copper, for example.

As mentioned in the introductory remarks in the specification, the lossyresonant elements 24 are utilized to eliminate spurious oscillations ina traveling wave tube such as depicted in FIG. 1. The mechanism by whichspurious oscillations are eliminated will be described in more detailhereinafter.

Directing your attention to FIG. 7, there is depicted therein an w-,6diagram in which the fundamental mode of operation characteristic A isillustrative of the pass band for the cloverleaf circuit depicted inFIG. 1. As can be seen when 1r phase shift between sections occurs thefundamental mode A has a low or zero group velocity and is thereforesusceptible to resonant circuit oscillations for this particular mode.These oscillations at the band edge or low group velocity portion of thefundamental mode A are induced when the beam voltage represented bycharacteristic B sweeps through or is synchronized with the band edges.Such conditions of synchronization between the fundamental mode phasevelocity and the beam velocity can occur for a number of reasons.Characteristic B may also be said to be representative of the phasevelocity for the fundamental mode at the band edge.

Thus, it is apparent that synchronization between the beam voltage andphase velocity occurs at the band edge of the fundamental mode A whenthe beam voltage is either sweeping through its transient range to itsoperating velocity, as for example, characten'stic C, under pulseconditions or when overdriven R.F. energy slows the beam velocitythrough extraction of beam energy to a point where synchronizationbetween the phase velocity of the RF. energy of the fundamental mode atthe band edges and the beam velocity occurs. It is to be noted uponexamination of the diagram of FIG. 7 that the band edge is a relativelysmall portion of the passband of the cloverleaf structure. Thus, if onecould suppress propogation in that part of the fundamental mode which isencompassed by the frequency range denoted by, for example, the portionof the passband delineated by D in FIG. 7 one would effectivelyeliminate the possibility of oscillations occurring over this region.

The present invention provides a novel approach to this problem in thefollowing manner. Since the frequency range wherein band edgeoscillations can be induced is relatively small as can be seen uponexamination of FIG. 7, it is conceivable that selective loading can beaccomplished to load down this range of frequencies. Utilization of aninternally'disposed non-radiating resonant lossy element to accomplishthe purpose is taught by the present invention. Directing your attentionto FIG. 3 there is depicted an alternative embodiment of the presentinvention. In this embodiment a resonant cavity 27 is formed within thechamber 25 defined by the sinuous side walls 22 of the cloverleaf.Positioning of the cavity 27 is such that higher order modes aremaximally effected by physical perturbation thereof While thefundamental mode is minimally effected through physical perturbation. Aslot 28 is provided at the central portion of the cavity 27, the cavity27 is loaded with lossy material, such as, for example, Kanthal alloy Acomprising 5% aluminum, 22% chromium, 0.5% cobalt, the balance iron. Thelossy coating can be flame sprayed over the interior surfaces of thecavity 27 to a depth of approximately 0.005".

FIG. 5 depicts another alternative embodiment employing the lossyresonant element techniques as broadly disclosed by the presentinvention. The embodiment of FIG. 5 utilizes a coaxial resonator 29disposed in the chamber 25 which is defined by the same side walls 22 ofthe cloverleaf. A lossy material can be deposited on the interiorportions of the coaxial resonator in the same fashion as described withregard to FIGS. 3 and 4. Direct your attention once again to FIGS. 1 and2, the lossy resonant U-shaped loop elements 24 are preferably 0.050" indiameter copper wires sprayed with the aforementioned Kanthal alloy Aand bent into hairpin-like or U shapes brazed into the cloverleafsection 25 as shown. In a preferred embodiment the lossy modesuppressors 24 are disposed in substantial longitudinal alignment takenin the direction of the longitudinal axis of the tube. In addition, theplanes of the loops 24 are parallel to the longitudinal axis of thetube. In a preferred embodiment the radial extent of each loop is variedin each of the chambers 25 to thereby tune the resonant frequency of theloop to slightly different frequencies in successive chambers 25. Thelossy resonant loops 24 have their frequencies tuned to overlap thefrequency range where band edge oscillations are expected such as thatregion defined by d-d in FIG. 7.

In a typical S band tube of the present invention the upper edge of thepass band of the tube was at 2900 megacycles and the band edgeoscillations were observed to occur at 302.0 megacycles without theprovision of the mode suppressors 24. The mode suppressors 24 were tunedto blank the frequency range of the band edge oscillations and inparticular the radial extent of the loops in adjacent sections were 1%",1%", 1 /2", 1 /3" and 1%" and the band edge oscillations were found tobe completely suppressed for a tube such as depicted in FIG. 1.

Although a preferred embodiment utilizes a plurality of lossy resonantloops disposed in substantial longitudinal alignment taken in thedirection of the logitudial axis of the tube, it is to be understoodthat the present invention is not restricted to this particularembodiment or orientation. For example, the lossy resonant elements,such as the U-shaped loops 24 depicted in FIG. 2, may be advantageouslyutilized in each of the four chambers 25 defined by the sinuous sidewall portions 22 of the cloverleaf in each section of the cloverleaf.Furthermore, the plane of the loops, although preferably disposedparallel to the longitudinal axis of the tube, can be varied therefromwithout departing from the scope of the present invention. The effect ofsuch a displacement of the plane of the lossy resonant elements is toreduce the coupling between the RF. energy and the loops. Maximum coupling to a given electromagnetic field configuration occurs when theplane of the loop is parallel to the E- fields and perpendicular to theH-fields.

Experimental evidence has shown that the radial extent of the loopswithin the cavities primarily determines the resonant frequency of theloop. A simple technique for determining the frequency at which the loopis resonating is as follows: A cold test cavity may be utilized with asignal generator transmitting R.F. energy at the particu lar frequencyof interest into the cavity. A standard crystal detector may be disposedat the output portion of the cavity. The output signal from the detectormay be applied to an oscilloscope and observed thereon. At a minimum oftransmission, at the undesired frequency, it is readily apparent thatenergy at this frequency is being dissipated within the cavity and notpropagated therethrough. Therefore, loops having varying radial extents,element or loop diameters and shapes in a particular chamber or chambersof a cloverleaf section or sections can be employed to pre-determine theparticular frequency range which is to be suppressed. For example,assume the desired frequency to besuppressed is at approximately 3000megacycles. A loop having a particuler diameter and radial extent andconfiguration may be intubeoper-ation. This technique is obviouslyextendible to blanket any desired range of undesired frequencies whileminimizing perturbation of the operating mode.

If very heavy loading of a particular frequency is desired,thenobviously one could position aloop in each of the chambers 25 defined bythe side Wall portions 22 of the cloverleaf wherein each of the loopswould be of identical physical shape and thus resonant at the samefrequency. Alternatively, if a broad spectrum is desired to be blanketedsuch as frequencies above the passband of the tube, then quite obviouslydifferent sized loops reso-v nant at different frequencies canadvantageously be positioned in each of the chambers 25 defined by theside wall portions 22 along the entire longitudinal extent of the slowwave circuit. If both the band edge portion of the fundamental mode andhigher order modes are desired to be. suppressed, then obviouslyutilizing the techniques of the present invention, one wouldadvantageously employ resonant loops in such a fashion that the entirespectrum including the upper edge of the fundamental mode and undesiredfrequencies thereabove would be blanketed. In the preferred embodimentof the present invention, however, the lossy resonant loops are tuned toblanket the band edge of the operating mode and in addition since theplane of the loops is parallel to the E-fields and perpendicular totheH-fields of higher order modes such as those represented by the,dot-dash lines 26' in FIG. 2 as well as being located at points ofmaximum field intensity of these modes, physical perturbation of thesemodes results in the destruction of the resonant circuit and periodicproperties thereof for a given mode which precludes oscillation in thesemodes. It is to be understood that higher order modes can be dissipatedby positioning lossy resonant elements in the cavities which are tunedto the frequency of oscillationof these modes and destroying these modesby dissipation techniques such as used to suppress ocillations at theband edge of the fundamental mode as 1 well as by physically perturbingthe electromagnetic fields of these modes by positioning the conductiveloops, cavities, etc., of the preferred embodiment in the vicinityofmaximum electromagnetic field intensity of higher order modes anddestroying them through physical perturbation of the fields in a givenportion or portions of the higher order mode while simultaneouslysuppressing the band edgeportion of the fundamental mode throughdissipation by'means of energy transfer to the lossy resonant element.

As can be seen upon examinationof FIG. 2, coupling between the R.F.;energy in each cloverleaf section and the resoant loop is primarilyinductive. The fundamental mode R.F. energy magnetic field portionthereof as represented by 26 is minimal at the peripheral portion 32 ofthe cloverleaf side walls .22. Therefore, energy extraction from thefundamental mode is minimal and the operating characteristics of thetube with regard to the fundamental mode are not seriously adverselyeffected over the. operating portion. However, certain higher ordermodes such as depicted in FIG. 2 have maximum H-fields in the vicinityof the loop 24 and will therefore, tend to couple very strongly theretoif the lossy resonant element or elements are designed to resonant atthe frequency of oscillation of the higher order modes and thus bedestroyed through dissipation or to be destroyed through physicalperturbation of the mode pattern by the presence of the conductiveelement even though dissipat-ion is minimal due' to coupling theretounder resonant conditions as mentioned above.

In FIG. 3, as mentioned previously, a resonantcavity 27 is disposed inthe chambers 25. The particular dimensions and resonant frequency of thecavity 27 defined by the peripheral wall portion 32 of the side wallportions of cloverleaf 22 and a metallic member such as copper septum 29can be determined through utilization of a signal generator and detectorsystem as previously explained. The particular frequencies desired to besuppressed can be modified as chosen with regard to the particularproblems presented by the chosen mode of operation. A greaterperturbation of the fundamental mode takes place when the cavityconfiguration depicted in FIG. 3 is employed due to increased physicalperturbation of the E-fields and the H-fields of the fundamental mode.However, the operating portion of the fundamental mode will not coupleinto the resonant chamber 27 since chamber 27 is effectively cut off forthe fundamental mode. The slot 28 through which energy couples into theresonant chamber 27 is advantageously positioned at the center portionof the septum 29 although obviously it could be varied at will. The slotdimensions 28 Will determine the cut-off frequency of the energypropagated therethrough. A particular example for slot dimensions wouldbe to so dimension the slot so that all frequencies below the upper bandedge of the fundamental mode are precluded from propagating therein. Thelossy material utilized within the cavity is preferably Kanthal A or anyother equivalent lossy material.

FIG. 5 utilizes a coaxial resonator 33 positioned in chamber 25 asdefined by side Wall portion 22 of the cloverleaf section. The radialextent of the coaxial resonator 33 is preferably AA at the frequency atwhich it is designated to suppress. The orientation number and radialextent of the coaxial resonator-s depicted in FIG. 5 can be varied atwill in order to blanket a particular frequency spectrum or suppresscertain undesired frequencies. The lossy material utilized within thecoaxial cavity resonator is preferably Kanthal A or any other equivalentlossy material.

FIG. 8 is an illustrative example of pulse-induced oscillations causedby the beam voltage sweeping through an illustrative operating rangeincluding the transient portions thereof. Characteristic F isrepresentative of a typical beam voltage pulse. Characteristics G and Hare typical examples of pulse-induced, rabbit-ear, oscillationsappearing when a beam pulse is introduced without employing theselective loading techniques of the present invention. It is apparentthat since it is, practically speaking, impossible to obtain a zero risetime that the voltage of the beam pulse as it sweeps up will synchronizeat some point with the band edge of the fundamental mode and at thispoint or points in the frequency spectrum a spurious pulse is produced.These rabbit-ear or pulse induced oscillations can occur both at thebeginning and end portions of a voltage pulse such as shown in FIG. 8.With utilization of the lossy resonant loading techniques of the presentinvention such that the band edge of the fundamental mode is blanketedthe rabbit-ear or pulse induced oscillations represented by G and H inFIG. 8 are completely suppressed and cannot be observed.

FIG. 9 is illustrative of a typical peak power out (P versus time (t)R.F. output characteristic for a traveling Wave tube such as depicted inFIG. 1..Characteristic z is representative of the RF. power outputobserved for a 2700 to a 2900megacycle passband over a 6.6 to 8microseconds duty cycle period. Characteristic i is an idealizedcondition and is practically speaking unobtainable with conventionalcloverleaf circuits even with ideal variable drive energy.Characteristic j is illustrative of a typical R.F. output spectrum when,for example 4 db overdriven conditions exist. Characteristics h, h andh" are representative of spurious oscillations occurring both at theband edges and within the passband itself at around 3020 me. without theresonant lossy circuit loading of the present invention. The h, h"oscillations are representative of band edge overdriven or pulse inducedoscillations while h within the passband is representative ofoscillations occurring at band edge frequency during overdrivenconditions only.

FIG. 10 depicts peak RF. power output (P vs. time (t) again utilizing a6 to 8 microsecond pulse duty cycle when the resonant lossy loading ofthe present invention is employed to blanket the band edge of thefundamental mode of operations. It is readily apparent that a flatoutput characteristic k at 4 db overdriven conditions is obtainedwithout the presence of spuroius oscillations both within thefundamental operating mode and the band edges thereof. The employment ofthe lossy resonant loading techniques of the present invention haseffectively completely eliminated drive induced oscillations with theresultant beneficiial advantage that the flat saturation characteristicsof the cloverleaf slow wave circuit can be utilized to maintain highoutput power over such as, for example, 8 to 9% bandwidths, and better,with a constant drive power. The variation of drive required to maintainthe desired stable power output in previous cloverleaf circuits is nolonger necessary with the utilization of the lossy resonant circuitstructures of the present invention since the tube can be driven pastsaturation to advantageously utilize the fiat saturation characteristicsthereof.

Although the utilization of lossy resonant circuits has been describedwith respect to the cloverleaf circuit in FIGS. 1-6 it is readilyapparent that the techniques of the present invention are applicable toother coupled cavity types of slow wave circuits. For example, a longslot coupled circuit such as described in Stanford University MicrowaveLaboratory, W. W. Hansen Laboratory of Physics, 1st and 2nd'AnnualReport for Period July 1958 to June 1960, entitled, Development of HighPower Broadband Tubes and Related Studies, under Air Force Contract AF301 (602)-1844, published January 1961, pages 93-124 can advantageouslybenefit from the use of the present invention. Another slow-wave circuitcalled by terminology, the centipede circuit discussed and described ina paper by M. Chodorow, A. F. Pearce, and D. K. Winslow, entitled, TheCentipede High Power Traveling Wave Tube, ML Report No. 695, MicrowaveLaboratory, Stanford University, May 1960, may advantageously benefitfrom the use of the lossy resonant circuit techniques of the presentinvention. Other coupled cavity slow-wave circuits may similarly benefitfrom the present invention as mentioned previously. In each case theparticular type of lossy resonant element utilized is a matter of choiceas well as the frequency range or ranges which are desired to beblanketed in order to suppress undesired oscillations. Regardless of thetype of slow wave circuit employed, the present invention is easilyapplied thereto by utilizing a frequency generator, oscilloscope anddetector arrangement, as described previously in conjunction withconventional mode configuration identification techniques. See also mycopending application Ser. No. 334,496 filed Dec. 30, 1963, togetherwith Robert L. Perry in which the lossy resonant loops of the presentinvention are utilized in the cloverleaf section of the [novel hybridtube described therein.

Since many changes would be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing 10 from the scope thereof, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense;

What is claimed is:

1 A high frequency electron discharge device including a traveling waveinteraction region and including: means for forming a stream ofelectrons, slow-wave interaction circuit means disposed along saidstream for electromagnetic interaction with said stream of electrons,and means disposed at the downstream end of said interaction circuitmeans for collecting said electron beam, said slow wave interactioncircuit means being disposed along said stream path for providingcumulative interaction between the stream of electrons and wave of RF.energy moving on said slow wave interaction circuit, said slow waveinteraction circuit means being of the coupled cavity type having atleast one lossy resonant element disposed therein internally of at leastone of the cavities of said coupled cavity slow-wave interaction circuitmeans, said at least one lossy resonant element being radially displacedfrom said stream of electrons and asymmetrical with respect to saidstream of electrons, said lossy resonant element adapted and arranged todissipate R.F. energy present on said slow-wave interaction circuit atat least one frequency, said at least one internally disposed lossyresonant element being physically disposed within the electromagneticfield pattern of the operating mode of the device within said at leastone cavity.

2. A device as defined in claim 1 wherein said lossy resonant element isa conductive wire loop having a lossy coating deposited thereon.

3. The device as defined in claim 1 wherein said lossy resonant elementis a resonant cavity having lossy material disposed therein.

4. A device as defined in claim 1 wherein said lossy resonant element isa coaxial cavity having a lossy coating deposited therein.

5. The device as defined in claim 1 wherein said coupled cavity slowwave interaction circuit means comprises a plurality of coupledcloverleaf type slow wave circuit sections and wherein at least two ofsaid plurality of coupled cloverleaf slow wave circuit sections havelossy resonant elements internally disposed therein.

6. The device as defined in claim 5 wherein said lossy resonant elementsare disposed in each of said cloverleaf sections at points of lowelectromagnetic field intensity of the fundamental mode of propagation.

7. The device as defined in claim 6 wherein said lossy resonant elementsare aligned in the same axial plane extending along the longitudinalaxis of the slow wave circuit.

8. The device as defined in claim 1 wherein said lossy resonant elementis disposed within a cavity of said coupled cavity slow-wave interactioncircuit such as to suppress oscillation in primarily by physicalperturbation of the electromagnetic fields of said higher order mode.

9. The device as defined in claim 1 wherein said lossy resonant elementis resonant at a frequency in the upper bandedge region of the operatingmode such as to suppress device oscillations at the upper band edge ofthe operating mode of the device.

10. The device as defined in claim 1 wherein said slow wave circuit is acloverleaf type slow wave circuit and wherein a plurality of resonantlossy elements are disposed along the longitudinal axis of said slowwave circuit, said lossy elements being conductive loops having lossycoatings deposited thereon and wherein said loops vary in radial extenttaken along the longitudinal axis of said slow wave circuit.

11. A high frequency electron discharge device having means for formingand projecting an electron beam along a predetermined electron beam axisdisposed at one end of said beam axis, slow-wave interaction circuitmeans disposed along said beam axis and electron beam collecting meansdisposed at the downstream end of said elecas to suppress deviceoscillations at certain frequencies both within and without the passbandof said device, said internally disposed lossy resonant elements beingphysically disposed within the electromagnetic field pattern of theOperating mode of the device within said cavities.

12. The device as defined in claim 11 wherein said lossy resonantelements are positioned in each of said coupled resonant cavities in aregion of minimal field strength for electromagnetic wave energy in theoperating mode of the device.

13. The device as defined in claim 12 wherein said lossy resonantelements are resonant within a frequency band to dissipateelectromagnetic energy at frequencies in the vicinity of the band edgeof the operating mode of said device.

14. The device as defined in claim 13 wherein said coupled cavitiesinclude lossy resonant elements tuned to suppress higher order moderesonant circuit oscillations of said device occurring at frequenciesabove the passband of the operating mode.

15. The device as defined in claim 11 wherein said lossy resonantelements are conductive loops having lossy material deposited thereon.

16. The device as defined in claim 11 wherein said lossy resonantelements are resonant cavities having lossy material deposited therein.

17. The device as defined in claim 11 wherein said lossy resonantelements are coaxial cavities having lossy material deposited therein.

18. A high frequency electron discharge device of the traveling wavetube type having a vacuum envelope and having a coupled cavity type ofslow wave circuit disposed therein, said coupled cavity type of slowwave circuit having a plurality of lossy resonant elements disposedtherein, and protruding from the cavities side walls into the. interiorregion of the volume defined by the cavities of said coupled cavity slowwave circuit, said lossy resonant elements being adapted and arranged toabsorb electromagnetic wave energy at certain frequencies withoutradiating electromagnetic energy external to the slow wave circuit saidlossy resonant elements being physically disposed within theelectromagnetic field pattern of the operating mode of the device.

19. The device as defined in claim 18 wherein said lossy resonantelements are adapted and arranged to suppress oscillations of saiddevice at the band edge of the operating mode of said device primarilythrough dissipation and wherein said lossy resonant elements are alsoadapted and arranged to suppress oscillations of said device in a higherorder mode primarily through physical perturbation of said higher ordermode.

20. A high frequency electron discharge device comprising an electrongun, slow wave interaction circuit and collector means physicallyattached in an operative relationship such that high frequencyelectromagnetic energy propagated along said slow wave interactioncircuit will cumulatively interact with an electron beam emanating fromsaid electron gun and directed along an elongated control, said deviceincluding conductive loops positioned within said slow wave circuit,said conductive loops being radially removed from said central beam axisand individually asymmetrical with respect to said central beam axis,said conductive loops being physically disposed within theelectromagnetic field pattern of the operating mode of the device withinsaid slow wave circuit.

21. The device as defined in claim 20 wherein said conductive loops areadapted and arranged to dissipate electromagnetic energy at certainfrequencies of order to prevent oscillations in said device fromoccurring at said frequencies.

References Cited UNITED STATES PATENTS 2,785,381 3/1957 Brown 333-982,841,738 7/1958 Pierce 315-35 2,952,795 9/1960 Craig et al 315-352,970,242 1/1961 Jepsen 315-539 3,181,024 4/1965 Sensiper 315-353,221,204 1 1/1965 Hent et al. 315-35 3,221,205 11/1965 Sensiper 315-35HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

S. CHATMON, JR., Assistant Examiner.

1. A HIGH FREQUENCY ELECTRON DISCHARGE DEVICE INCLUDING A TRAVELLINGWAVE INTERACTION REGION AND INCLUDING: MEANS FOR FORMING A STREAM OFELECTRONS, SLOW-WAVE INTERACTION CIRCUIT MEANS DISPOSED ALONG SAIDSTREAM FOR ELECTROMAGNETIC INTERACTION WITH SAID STREAM OF ELECTRONS,AND MEANS DISPOSED AT THE DOWNSTREAM END OF SAID INTERACTION CIRCUITMEANS FOR COLLECTING SAID ELECTRON BEAM, SAID SLOW WAVE INTERACTIONCIRCUIT MEANS BEING DISPOSED ALONG SAID STREAM PATH FOR PROVIDINGCUMULATIVE INTERACTION BETWEEN THE STREAM OF ELECTRONS AND WAVE OF R.F.ENERGY MOVING ON SAID SLOW WAVE INTERACTION CIRCUIT, SAID SLOW WAVEINTERACTION CIRCUIT MEANS BEING OF THE COUPLED CAVITY TYPE HAVING ATLEAST ONE LOSSY RESONANT ELEMENT DISPOSED THEREIN INTERNALLY OF AT LEASTONE OF THE CAVITIES OF SAID COUPLED CAVITY SLOW-WAVE INTERACTION CIRCUITMEANS, SAID AT LEAST ONE LOSSY RESONANT ELEMENT BEING RADIALLY DISPLACEDFROM SAID STREAM OF ELECTRONS AND ASYMMETRICAL WITH RESPECT TO SAIDSTREAM OF ELECTRONS, SAID LOSSY RESONANT ELEMENT ADAPTED AND ARRANGED TODISSIPATE R.F. ENERGY PRESENT ON SAID SLOW-WAVE INTERACTION CIRCUIT ATAT LEAST ONE FREQUENCY, SAID AT LEAST ONE INTERNALLY DISPOSED LOSSYRESONANT ELEMENT BEING PHYSICALLY DISPOSED WITHIN THE ELECTROMAGNETICFIELD PATTERN OF THE OPERATING MODE OF THE DEVICE WITHIN SAID AT LEASTONE CAVITY.